OSMOSIS

• EFFECT OF CONCENTRATION ON MASS

  Scientists investigate the effects of osmosis on living cells. By observing with a microscope or measuring changes in cylinders or discs of fresh potato or beetroot.

  Cylinders will have a larger mass than discs, so scientists will have larger measurements to work with.
• AIM

  • To investigate the effect of a range of salt solutions, on the mass of potato cylinders. Concentration is measured in moles.
  • To determine the concentration of the cell sap of potato cells.
  • Independent Variable: The concentration of the sucrose solutions, with a range of 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm-3.
  • Dependent variable: The change in mass of the potato cylinders.
  • Control variables: The time that each cylinder is left in the sucrose solution, the size of each cylinder.
• METHOD:

  1. Set up 6 boiling tubes containing the same volume of one of the sodium chloride solutions label based on concentrations.
  2. Prepare 6 potato cylinders using a borer at the same length. Dry each potato cylinder using a paper towel to remove excess liquid and record its mass
  3. Leave the potato cylinders in the boiling tubes for 40 minutes.
  4. Remove each potato cylinder from its boiling tube, remove excess liquid from the surface of the cylinder with a paper towel and record its mass.
  5. Repeat the experiment to obtain multiple values of mass change for each solution and calculate a mean
• • RISKS
  • Make sure that the potato is placed on a ceramic tile when using the cork borer – do not cut the potato cylinders towards your hand.
  • Care must be taken when using the scalpel.
  • Wear eye protection when using chemical solutions.
• This experiment shows the effect of osmosis on plant tissue. The cylinders will decrease or increase in mass if they lose or gain water by osmosis.
• RATE OF WATER UPTAKE:

  Rate of water uptake= change in mass x (60 minutes / time in measure in minutes)
• PERCENTAGE CHANGE IN MASS:

  • Some variation in mass between the potato cylinders at the beginning of the experiment as it would be impractical to prepare the cylinders so that they were identical in mass.
  • change in mass=(mass at end−mass at start) / mass at the start ×100
• ANALYSING RESULTS:

  • Percentage changes in mass must be calculated for each cylinder.
  • A mean value for the change in mass of potato cylinders at each concentration - measured in moles of sucrose per dm3 of solution, or mol dm−3 – should be calculated.
  • A graph is plotted of change in mass, in per cent, against concentration of sucrose.
  • Where potato cylinders have gained in mass, the change will be positive.
  • Where potato cylinders have decreased in mass, the change will be negative.
• ANALYSING RESULTS:

  • Where the line crosses the horizontal axis of the graph of results at 0 per cent change in mass, the sodium chloride concentration is equal to the concentration of the contents of the potato cells.
  • Therefore, the concentration of the water in the outer solution is equal to the concentration of the water inside the cells.
  • This can be identified on the graph as the point which shows no change in mass, and therefore represents no net movement of water by osmosis.